<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="https://cdn.jsdelivr.net/npm/vue@2.7.14/dist/vue.js"></script>
</head>
<body>
    <div id="app">
        <button @mouseenter = msg++>+</button>
        <span>{{msg}}</span>
        <button @mouseenter = msg-->-</button>
    </div>
    <script>
        const app  = new Vue({
            el : "#app",
            data:{
                msg : 100
            }
        })
    </script>
</body>
</html>